Implicit certificate scheme

ABSTRACT

A method of generating a public key in a secure digital communication system, having at least one trusted entity CA and subscriber entities A. For each entity A, the trusted entity selects a unique identity distinguishing the entity A. The trusted entity then generates a public key reconstruction public data of the entity A by mathematically combining public values obtained from respective private values of the trusted entity and the entity A. The unique identity and public key reconstruction public data of the entity A serve as A&#39;s implicit certificate. The trusted entity combines the implicit certificate information with a mathematical function to derive an entity information ƒ and generates a value k A  by binding with ƒ with private values of the trusted entity. The trusted entity transmits the value k A  to the entity to permit A to generate a private key from k A , A&#39;s private value and A&#39;s implicit certificate. The entity A&#39;s public key information may be reconstructed from public information, and A&#39;s implicit certificate.

This application is a continuation of U.S. patent application Ser. No. 09/667,817 filed on Sep. 22, 2000, now U.S. Pat. No. 6,792,530 which is a continuation of PCT Application No. PCT/CA99/00244 filed on Mar. 23, 1999 which claims priority from Canadian Patent Application No. 2,235,359 filed on Apr. 20, 1998 and Canadian Patent Application No. 2,232,936 filed on Mar. 23, 1998.

This invention relates to key distribution schemes for transfer and authentication of encryption keys.

BACKGROUND OF THE INVENTION

Diffie-Hellman key agreement provided the first practical solution to the key distribution problem, in cryptographic systems. The key agreement protocol allowed two parties never having met in advance or shared key material to establish a shared secret by exchanging messages over an open (unsecured) channel. The security rests on the intractability of the Diffie-Hellman problem and the related problem of computing discrete logarithms.

With the advent of the Internet and such like the requirement for large-scale distribution of public keys and public key certificates are becoming increasingly important. Public-key certificates are a vehicle by which public keys may be stored, distributed or forwarded over unsecured media without danger of undetectable manipulation. The objective is to make one parties' public key available to others such that its authenticity and validity are verifiable.

A public-key certificate is a data structure consisting of a data part and a signature part. The data part contains cleartext data including as a minimum, public key and a string identifying the party to be associated therewith. The signature part consists of the digital signature of a certification authority (CA) over the data part, thereby binding the entities identity to the specified public key. The CA is a trusted third party whose signature on the certificate vouches for the authenticity of the public key bound to the subject entity.

Identity-based systems (ID-based system) resemble ordinary public-key systems, involving a private transformation and a public transformation, but parties do not have explicit public keys as before. Instead, the public key is effectively replaced by a party's publicly available identity information (e.g. name or network address). Any publicly available information, which uniquely identifies the party and can be undeniably associated with the party, may serve as identity information.

An alternate approach to distributing public keys involves implicitly certified public keys. Here explicit user public keys exist, but they must be reconstructed rather than transported by public-key certificates as in certificate based systems. Thus implicitly certified public keys may be used as an alternative means for distributing public keys (e.g. Diffie-Hellman keys).

An example of an implicitly certified public key mechanism is known as Gunther's implicitly-certified (ID-based) public key method. In this method:

-   -   1. A trusted server T selects an appropriate fixed public prime         p and generator α of Z^(•) _(p). T selects a random integer t,         with 1≦t≦p−2 and gcd(t, p−1)=1, as its private key, and         publishes its public key u=α^(t) mod p, along with α, p.     -   2. T assigns to each party A a unique name or identifying string         I_(A) and a random integer k_(A) with gcd(k_(A), p−1)=1. T then         computes P_(A)=α^(K) ^(A) mod p. P_(A) is A's KEY reconstruction         public data, allowing other parties to compute (P_(A))^(a)         below.     -   3. Using a suitable hash function h, T solves the following         equation for a:         H(I _(A))≡t·P _(A) +k _(A) a (mod p−1)     -   4. T securely transmits to A the pair (r, s)=(P_(A), a), which         is T's ElGamal signature on I_(A). (a is A's private key for         Diffie-Hellman key-agreement)     -   5. Any other party can then reconstruct A's Diffie-Hellman         public key P_(A) ^(a) entirely from publicly available         information (α, I_(A), u, P_(A), p) by computing:         P _(A) ^(a)≡α^(h(I) ^(A) ⁾ u ^(−P) ^(A) mod p

Thus for discrete logarithm problems, signing a certificate needs one exponentiation operation, but reconstructing the ID-based implicitly-verifiable public key needs two exponentiations. It is known that exponentiation in the group Z^(•) _(p) and its analog scalar multiplication of a point in E(F_(q)) is computationally intensive. For example an RSA scheme is extremely slow compared to elliptic curve systems, However despite the resounding efficiency of EC systems over RSA type systems this is still a problem particularly for computing devices having limited computing power such as “smart cards”, pagers and such like.

SUMMARY OF THE INVENTION

In one aspect, there is provided a computer readable medium comprising a value k_(A) generated by an entity in a secure communication system to be used in generating a private key of a subscriber A, the value k_(A) being generated by binding entity information f with private values of the entity, the information f being derived by combining information in an implicit certificate in accordance with a mathematical function, the implicit certificate being comprised of a unique identity I_(A) and public key reconstruction data γ_(A) generated by mathematically combining public values obtained from respective private values of the subscriber A and the entity, wherein the private key can be generated from the value k_(A), a private value of the subscriber A, and the implicit certificate.

In another aspect, there is provided a cryptographic unit of an entity in a secure communication system for enabling at least one subscriber A to generate a public key, the cryptographic unit being configured for: selecting a unique identity I_(A) distinguishing the subscriber A; generating public key reconstruction data γ_(A) of the subscriber A by mathematically combining public values obtained from respective private values of the entity and the subscriber A, to obtain a pair (I_(A), γ_(A)) serving as the subscriber A's implicit certificate; combining the implicit certificate (I_(A), γ_(A)) in accordance with a mathematical function F(I_(A), γ_(A)) to derive entity information f; and generating a value k_(A) by binding the entity information f with private values of the entity and transmitting the value k_(A) to the subscriber A to permit the subscriber A to generate a private key from the value k_(A), the private value of the subscriber A, and the implicit certificate; whereby the subscriber A's public key may be reconstructed from public information, the public key reconstruction data γ_(A), and the identity I_(A).

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which:

FIG. 1 is a schematic representation of a first system configuration according to an embodiment of the present invention; and

FIG. 2 is a schematic representation of a second system configuration according to an embodiment in the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Referring to FIG. 1, a system with implicitly-certified public keys is shown generally by 10. This system 10 includes a trusted third party CA and at least a pair of first and second correspondents A and B respectively. The correspondents A and B exchange information over a communication channel and each includes a cryptographic unit for performing visual finding/verification and encryption/decryption.

Referring back to FIG. 1, the trusted party CA selects an appropriate prime p with p=tq+1 where q is a large prime and a generator α of order q. The CA selects a random integer c, with 1≦c≦q−1 as its private key, then computes the public key β=α^(c) mod p and publishes (β, α, p, q).

Scheme 1:

-   -   1. For each party A, the CA choose a unique distinguished name         or identity I_(A) (e.g., name, address, phone number), and a         random integer c_(A) with 1≦c_(A)≦q−1. Then the CA computes         γ_(A)=α^(c) ^(A) mod p. (γ_(A) is the party A's public key         reconstruction public data. The pair (I_(A), γ_(A)) serves as         A's implicit certificate)     -   2. The CA selects a function ƒ=F(I_(A), γ_(A)). For example,         F(γ_(A), I_(A))=γ_(A)+h(I_(A)), or F(γ_(A),         I_(A))=h(γ_(A)+I_(A)) where h is a secure hash function and         solves the following equation for a, which is party A's private         key. If a=0, then the CA chooses another c_(A) and resolves the         equation.         1=cf+c _(A) a (mod q)     -   3. The CA securely sends the triple (γ_(A), a, I_(A)) to A,         which is CA's signature on I_(A). Then         -   a is A's private key;         -   γ_(A) is A's generator; and         -   γ_(A) ^(a) (=α^(cAa)) is A's public key.     -    A publishes (α, I_(A), β, γ_(A), p, q) in the public domain.     -   4. Anyone can obtain party A's (ID-based) implicitly verifiable         public key from the public domain by computing,         γ_(A) ^(a)=αβ^(−f) (mod p),         thus deriving the public key from the above equation, which         requires only one exponentiation operation.

Although everyone can reconstruct party A's public key from public data, this does not mean that the reconstructed public key γ_(A) ^(a) has been certified. This scheme is more effective when it is combined with an application protocol that shows that party A has complete knowledge of the corresponding private key. For example, with the MQV key-agreement scheme or with any signature scheme and particularly with an KCDSA (Korean Certificate based Digital Signature Algorithm). In general, this implicit certificate scheme can be used with any scheme, which is required to verify the certificate. This may be demonstrated by referring to the Digital Signature Algorithm (DSA) signature scheme.

Suppose Alice has a private key a, generator γ_(A) and publishes (a, I_(A), β, γ_(A), p, q) in the public domain. Now Alice wants to sign a message M using DSA.

Alice does the following:

-   -   1. randomly chooses k, computes r=γ_(A) ^(k) (mod p);     -   2. computes c=sha−1 (M);     -   3. computes s=k⁻¹(e+ar) (mod p);     -   4. The signature on M is (r, s).

Verifier does the following:

-   -   1. gets Alice's public data (α, I_(A), β, γ_(A), p, q) and         reconstructs the public key         δ_(A)=γ_(A) ^(a)=αβ^(−f) (mod p);     -   2. computes e=sha-1(M);     -   3. computes u₁=es⁻¹ (mod q) and u₂=rs⁻¹ (mod q);     -   4. computes r′=γ_(A) ^(u) ¹ δ_(A) ^(u) ¹ mod p;     -   5. if r=r′, the signature is verified. At the same time Alice's         (ID-based) public key is implicitly verified.

The pair (I_(A), γ_(A)) serves as certificate of Alice. Reconstructing the public key serves as implicit verification when the application protocol results in a valid verification. Recall that obtaining the public key needs only one exponentiation operation.

In an alternate embodiment, the scheme can be generalized to most ElGamal signature schemes by modifying the signing equation appropriately. In the following section, we give some examples.

Scheme 2:

The CA uses the signing equation 1=ca+c_(A)ƒ (mod q). The CA securely sends the triple (γ_(A), a, I_(A)) to A, then a is A's private key, β is A's generator and β^(a) is A's public key. A publishes (α, I_(A), β, γ_(A), p, q) in public domain. Anyone can obtain A's (ID-based) implicitly certified public key from the public domain by computing β^(a)=αγ_(A) ^(−ƒ) (mod p)

For this scheme, each user has the same generator β which is the CA's public key.

Scheme 3:

The CA uses the signing equation a=cf+_(CA) (mod q). The CA securely sends the triple (γ_(A), a, I_(A)) to A, then a is A's private key, α is A's generator and α^(a) is A's public key. A publishes (α, I_(A), β, γ_(A), p, q) in the public domain. Anyone can obtain A's (ID-based) implicitly certified public key from the public domain by computing α^(a)=β_(γA) ^(f) (mod p)

For this scheme, each user including the CA has the same generator α.

Scheme 4:

The CA uses the signing equation a≡_(CA)f+c (mod q). The CA securely sends the triple (γ_(A), a, I_(A)) to A, then a is A's private key, α is A's generator and α^(a) is A's public key. A publishes (α, I_(A), β, γ_(A), p, q) in the public domain. Anyone can obtain A's (ID-based) implicitly certified public key from the public domain by computing α^(a)=γ_(A) ^(ƒ)β (mod p)

For this scheme, each user including CA has same generator α.

In the above schemes the user or party A does not have freedom to choose its own private key. The following schemes as illustrated in FIG. 2 both the CA and the user control the user's private key but only the user knows its private key.

Scheme 5:

A first randomly chooses an integer k and computes α^(k), then sends it to the CA. The CA computes γ_(A)=α^(kC) ^(A) mod p, and solves the following signing equation for k_(A) 1=cf+c _(A) k _(A) (mod q).

Then the CA computes γ_(A) ¹=α^(C) ^(A) mod p and sends the triple (γ_(A) ¹, k_(A), I_(A)) to A. A computes a=k_(A)k⁻¹ (mod q) and γ_(A)=(γ_(A) ¹)^(k) (mod p)). Then a is A's private key, γ_(A) is A's generator and γ_(A) ^(a) is A's public key. A publishes (α, I_(A), β, γ_(A), p, q) in the public domain. Anyone can obtain A's (ID-based) implicitly certified public key from the public domain by computing γ_(A) ^(a)=αβ^(−f) (mod p)

Scheme 6:

-   -   1. A randomly chooses an integer k and computes β^(k), then         sends it to the CA.     -   2. The CA randomly chooses an integer c_(A) computes         γ_(A)=β^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A)), solves the         signing equation for k_(A) (if k_(A)=0, then choose another         c_(A))         1=ck _(A) +c _(A)ƒ (mod q).

Then CA computers γ_(A) ¹=γ^(c) ^(A) ^(c) ⁻¹ (mod p) and sends (the triple (γ_(A) ¹, k_(A), I_(A)) to A.

Note: (γ_(A) ¹, k_(A), I_(A)) can be sent by public channel.

-   -   3. A computes γ_(A)=(γ_(A) ¹)^(k) ⁻¹ α^(k) (mod p), ƒ=F(γ_(A),         I_(A)), and a=k_(A)−kƒ (mod q). (if a=0, 1, then goes back to         step 1.). Then checks if β^(a)=αγ_(A) ^(−ƒ). Now a is A's         private key, β is A's generator and β^(a) is A's public key. A         publishes (α, I_(A), β, γ_(A), p, q) in the public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         β^(a)=αγ_(A) ^(−ƒ) (mod p)         Scheme 7:

A first randomly chooses an integer k and computes α^(k), then sends it to the CA. Now CA computes γ_(A)=α^(k)α^(CA) (mod p), solves the signing equation for k_(A) k _(A) ≡cf+c _(A) (mod q)

Then the CA computes γ_(A) ¹=(α^(k))^(CA) (mod p) and sends the triple (γ_(A) ¹, k_(A), I_(A)) to A. A computes γ_(A)=(γ_(A) ¹)^(k−1)α^(k) (mod p). Then a=k_(A)+k (mod q) is A's private key, α is A's generator and α^(a) is A's public key. A publishes (α, I_(A), β, γ_(A), p, q) in public domain. Anyone can obtain A's (ID-based) implicitly certified public key from the public domain by computing α^(a)=β^(f) _(γA) (mod p) Scheme 8:

-   -   1. A randomly chooses an integer k and computes α^(k), then         sends it to the CA.     -   2. The CA, randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A)), computes         k_(A) (if k_(A)=0, then choose another c_(A))         k _(A) ≡c _(A) ƒ+c (mod q).

Then CA computers γ_(A) ¹=(α^(k))^(c) ^(A) (mod p) and sends the triple (γ_(A) ¹, k_(A), I_(A)) to A.

Note: (γ_(A) ¹, k_(A), I_(A)) can be sent by public channel.

-   -   3. A computes γ_(A)=(γ_(A) ¹)^(k) ⁻¹ α^(k) (mod p), ƒ=F(γ_(A),         I_(A)), and a=k_(A)+kƒ (mod q). (if a=0, 1, then goes back to         step 1.). Then checks if α^(a)=γ_(A) ^(ƒ)β. Now a is A's private         key, α is A's generator and α^(a) is A's public key. A publishes         (α, I_(A), β, γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=γ_(A) ^(ƒ)β (mod p)

In the above schemes 5-8, anyone can get some partial information of user A's private key α since k_(A) is sent by public channel. To hide this information and to speed up computation of above schemes, we introduce DES encryption to get following scheme 9-12 by modifying scheme 5-8. The advantages in scheme 9-12 is that user can compute K easily since β is fixed.

Scheme 9:

-   -   1. A randomly chooses an integer k and computes α_(k), then         sends it to CA.     -   2. CA randomly chooses an integer C_(A), computes γ_(A)=α^(kc)         ^(A) (mod p) and ƒ=F(γA, β, I_(A)), solves the signing equation         for k_(A) (if k_(A)=0, then choose another C_(A)).         1=cƒ+c _(A) k _(A) (mod q)

Next CA computes K=(a^(k))^(c) (mod p) and k _(A)=DES_(K)(k_(A)), then sends the triple (γ_(A), k _(A), I_(A)) to A.

γ_(A)

-   -   3. A computes K=β^(k) (mod p), k_(A)=DES_(k)( k _(A)), and         a=k_(A)k⁻¹ (mod q). (if a=1, then goes back to step 1). Then         checks if γ_(A) ^(a)=αβ^(−ƒ). Now a is A's private key, γ_(A) is         A's generator and γ_(A) ^(a) is A's public key. A publishes (α,         I_(A), β, γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         γ_(A) ^(a)=αβ^(−ƒ) (mod p)         Scheme 10:     -   1. A randomly chooses an integer k and computes β^(k), then         sends it to CA.     -   2. CA randomly chooses an integer C_(A), computes         γ_(A)=β^(k)α^(C) ^(A) (mod p) and ƒ=F(γ_(A), β, I_(A)), solves         the signing equation for k_(A) (if k_(A)=0, then choose another         C_(A).         1=ck _(A) +c _(A)ƒ (mod q)

Next CA computes K=(β^(k))^(c) ^(u) ^(c) ⁻¹ =α^(kc) ^(A) (mod p) and k _(A)=DES_(K) (k_(A)), then sends the triple (γ_(A) k _(A), I_(A)) to A.

Note: (γ_(A) k _(A), I_(A)) can be sent by public channel.

-   -   3. A computes K=(γ_(A)/β^(k))^(k)=αkc ^(A) (mod p),         k_(A)=DES_(k)( k _(A)), ƒ=F(γ_(A), β, I_(A)) and computes         a=k_(A)−kƒ (mod q). (if a=0, 1, then goes back to step 1). Then         checks if β^(a)=αγ_(A) ^(−ƒ). Now a is A's private key, β is A's         generator and β^(a) is A's public key. A publishes (α, I_(A), β,         γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         β^(a)=αγ_(A) ^(−ƒ) (mod p)         Scheme 11:     -   1. A randomly chooses an integer k and computes a^(k), then         sends it to CA.     -   2. CA randomly chooses an integer C_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), β, I_(A)) computes         k_(A) (if k_(A)=0, then choose another C_(A))         k _(A) =cƒ+c _(A) (mod q).

Next CA computes K=(α^(k))^(c) (mod p) and k _(A)=DES_(K)(k_(A)), then sends the triple (γ_(A), k _(A), I_(A)) to A.

Note: (γ_(A), k _(A), I_(A)) can be sent by public channel.

-   -   3. A computes K=β^(k) (mod p), k_(A)=DES_(K)( k _(A)), and         a=k_(A)+k (mod q) (if a=0, 1, then goes back to step 1). Then         checks if α^(a)=β^(ƒ)γ_(A). Now α is A's private key, α is A's         generator and α^(a) is A's public key. A publishes (α, I_(A), β,         γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing α^(a)=γ_(A) ^(ƒ) (mod p)         Scheme 12:     -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer C_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), β, I_(A)) computes         k_(A) (if k_(A)=0, then choose another C_(A)) k_(A)=c_(A)ƒ+c         (mod q)

Next CA computes K=(α^(k))^(c) (mod p) and k _(A)=DES_(k)(k_(A)), then sends the triple (γ_(A), k _(A), I_(A)) to A.

Note: (γ_(A), k _(A), I_(A)) can be sent by public channel.

-   -   3. A computes K=β^(k) (mod p), k_(A)=DES_(k)( k _(A)),         ƒ=F(γ_(A), β, I_(A)), and a=k_(A)+kƒ (mod q). (if a=0, 1, then         goes back to step 1). Then checks if α^(a)=γ_(A) ^(ƒ)β. Now a is         A's private key, α is A's generator and α^(a) is A's public key.         A publishes (α, I_(A), β, γ_(A), p, q). Anyone can obtain A's         (ID-based) implicitly certified public key from the public         domain by computing         α^(a)=γ_(A) ^(ƒ)β (mod p)

The advantages for schemes 9-12 are that user A can compute K easily since β is fixed and that k_(A) is encrypted such that no other people can know it.

Note that for schemes 5-12, adding an option parameter OP to the function F(γ_(A), β, I_(A)) (i.e., ƒ=F(γ_(A), β, I_(A), OP) will make the schemes more useful. For example, OP=α^(a) ^(E) , where a_(E) is user A's private encryption key and α^(a) _(E) is user A's public encryption key. Following scheme 15 is a modification of scheme 7. Schemes 5-12 can be modified in the same way. The schemes 1-4 can also be modified in the same way.

Scheme 13:

-   -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP),         computes k_(A) (if k_(A)=0, then choose another c_(A))         k _(A) ≡cƒ+c _(A) (mod q).

Next CA computers K=H((α^(k))^(c)) and k _(A)=DES_(K)(k_(A)), then sends the triple (ƒ, k _(A), I_(A)) to A.

-   -   3. A computes K=H(β^(k)), k_(A)=DES_(K)( k _(A)), and a=k_(A)+k         (mod q) (if a=0, 1, then goes back to step 1.) Then computes         γ_(A)=α^(a) β^(−ƒ) (mod p) and checks if ƒ=F(γ_(A), I_(A), OP).         Now α is A's private key, α is A's generator and α^(a) is A's         public key. A publishes (α, I_(A), β, γ_(A), p, q) in public         domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=β^(ƒ)γ_(A) (mod p)

Furthermore we can reduce the bandwidth by following scheme 14.

Scheme 14:

-   -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and set {circumflex over (γ)}_(A)         as the first 80 least significant bits of γ_(A). Then computes         ƒ=F({circumflex over (γ)}_(A), I_(A), OP) and k_(A) (if k_(A)=0,         then choose another c_(A))         k _(A) ≡cƒ+c _(A) (mod q).

Next CA computers K=(α^(k))^(c) (mod p) and k _(A)=DES_(K)(k_(A)), then sends the triple ({circumflex over (γ)}_(A), k _(A), I_(A)) to A.

Note: ({circumflex over (γ)}_(A), k _(A), I_(A)) can be sent by public channel.

-   -   3. A computes K=β^(k) (mod p), k_(A)=DES_(K)( k _(A)), and         a=k_(A)+k (mod q) (if a=0, 1, then goes back to step 1.) Then         computes ƒ=F({circumflex over (γ)}_(A), β, I_(A)), γ_(A)=α^(a)         β^(−ƒ) (mod p) and checks if the first 80 least significant bits         of γ_(A) is {circumflex over (γ)}_(A). Now a is A's private key,         α is A's generator and α^(a) is A's public key. A publishes (α,         I_(A), β, γ_(A)p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=β^(ƒ)γ_(A) (mod p)

The security level of scheme 5.c is not as other schemes we discuss before. Scheme 5.c only has 80 bit security. But it is OK for practical application Now. We can extend the first 80 least significant bits to the half least significant bits of γ_(A).

The implicit certificate can be used to certify some other useful information by including the information in the option parameter OP. For example OP=α^(a) ^(E) ∥OP₂, where a_(E) is user A's another private key and α^(a) ^(E) is the corresponding public key. Following scheme 15 is a modification of scheme 7. Other schemes can be modified in the same way.

Scheme 15:

-   -   1. A randomly chooses an integer a_(E) and computes α^(a) ^(E) .     -   2. A randomly chooses an integer k and computes α^(k), then         sends α^(k) and α^(a) ^(E) to CA.     -   3. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), β, I_(A), α^(a)         ^(E) ), (for example, ƒ=F(γ_(A), β, I_(A), α^(a) ^(E)         )=h(γ_(A)∥β∥I_(A)∥α^(a) ^(E) )) computes k_(A) (if k_(A)=0, then         choose another C_(A))         k _(A) =cƒ+cA (mod q)

Then CA computes γ_(A) ¹=(α^(k))^(c) ^(A) (mod p) and sends the triple (γ_(A) ¹, k_(A), I_(A)) to A.

Note: (γ_(A) ¹, k_(A), I_(A)) can be sent by public channel.

-   -   4. A computes a=k_(A)+k (mod q). (if a=0, 1, then goes back to         step 1) and computes γ_(A)=γ_(A) ¹)^(k) ⁻¹ α^(k) (mod p). Then         checks if α^(a)=β^(ƒ)γ_(A). Now a is A's private signing key, α         is A's generator and α^(a) is A's public signing key, a_(E) is A         's private encryption key and α^(a) ^(E) is A's public         encryption key. A publishes ((α, α^(a) ^(E) , I_(A), β, γ_(A),         p, q) in public domain.     -   5. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=β^(ƒ)γ_(A) (mod p)

Notes: (for scheme 13-15)

-   -   1. The identity I_(A) may be chosen either by CA or by entity A     -   2. CA should authenticate the entity A. It can be done by the         method described in the note 2 of scheme 11.     -   3. (ƒ, k _(A), I_(A)) or ({circumflex over (γ)}_(A), k _(A),         I_(A)) or (γ_(A) ¹, k_(A), I_(A)) can be sent by public channel.

In our schemes, (α, γ_(A)) is CA's signature on A's ID I_(A), it was supposed to be known by public. But now, only user A knows the a. So when we use these schemes, we should make sure that in application protocol, user A knows his/her own private key. In other words, the application protocol must guarantee that A uses his/her private key in the computation.

The security of the new scheme depends on the signing equations. For example, in scheme 1, the signing equation is 1=cƒ+c _(A)a (mod q).  (1)

We are going to show that for some choice of the one way function F(γ_(A), I_(A)), the new scheme 1 is equivalent to DSA.

Let's consider CA, using DSA signing equation to sign A's identity I_(A). First CA randomly choose a c_(A) and compute γ_(A)=α^(cA) mod p, then CA uses a secure hash function h to computer h(I_(A)), finally CA solves following equation for s. h(I _(A))≡cγ _(A) +c _(A) s (mod q).  (2)

Now (γ_(A), s) is CA's signature on I_(A).

Multiply equation (2) by h(I_(A))⁻¹ we got 1≡cγ _(A) h(I _(A))⁻¹ +c _(A) sh(I _(A))⁻¹ (mod q)

Let F(γ_(A), I_(A))=γ_(A) h(I_(A))⁻¹ and replace sh(I_(A))⁻¹ by a in above equation we got the equation (1). Obviously, equation (2) is equivalent to equation (1) if F(γ_(A), I_(A))=γ_(A) h(I_(A))⁻¹. That means, if anyone can break the scheme using the signing equation (1), then he/she can break the scheme using the signing equation (2) which is DSA scheme.

Heuristic arguments suggest our new schemes are secure for suitable choice of F(γ_(A), I_(A)), where F(γ_(A), I_(A))=γ_(A) h(I_(A)) or r(γ_(A), I_(A))=h(γ_(A), I_(A)). Note F(γ_(A), I_(A)) can be some other format, for example when I_(A) is small, say 20 bits, but q is more than 180 bits, then we can use F(γ_(A), I_(A))=γ_(A)+I_(A). A disadvantage of the new schemes is all users and CA use the same field size. However this is the way that all ID-based implicitly certified public key schemes work, for example, Girault's RSA based Diffie-Hellman public key agreement scheme.

A further set of schemes may also be described as follows:

System setup: A trusted party CA selects an appropriate prime p with p=tq+1 where q is a large prime and a generator α of order q. CA selects a random integer c, with 1<c<q as its private key, computes the public key β=α^(c) mod p and publishes (β, α, p, q). Then CA chooses a special cryptographic function ƒ=F(γ_(A), I_(A), OP) (ƒ: {0, 1}* →{1, 2, . . . (q−1)}) such that with this function, the signature scheme which used to produce implicit certificate is secure, where OP represents some option parameters that user may concern (such as date, or βthe CA's public key). For example, let h be a secure hash function, f can be one of following format

-   -   1. F(γ^(A), I_(A), OP)=γ_(A)+β+h(I_(A))     -   2. F(γ_(A), I_(A), OP)=h(γ_(A∥β∥I) _(A))     -   3. F(γ_(A), I_(A), OP)=γ_(A)+β+I_(A) where I_(A) has some         pattern (or when I_(A) is small, say 20 bits, and q is more than         180 bits)     -   4. F(γ_(A), I_(A), OP)=γ_(A)+h(I_(A))     -   5. F(γ_(A), I_(A), OP)=h(γ_(A)∥I_(A))     -   6. F(γ_(A), I_(A), OP)=γ_(A)+I_(A) where I_(A) has some pattern         (or when I_(A) is small, say 20 bits, and q is more than 180         bits)     -   7. It is very easy to change the parameters a little bit to get         a secure signature scheme from a given secure signature scheme.         So F(γ_(A), I_(A), OP) can be any other format that guarantee         the signature scheme which used to produce implicit certificate         is secure. Note that by suitable choosing F(γ_(A), I_(A), OP),         Any Elgamal-like signature scheme we know so far is equivalent         to one of the 4 families schemes we proposed in this paper if it         is used as implicit certificate scheme after modification. But         our proposed schemes have the most efficiency.

Note: the above system setup will be assumed in the following schemes.

Scheme 1.a:

-   -   1. For each entity A, CA chooses a unique distinguished name or         identity I_(A) (e.g., name, address, phone number), and a random         integer c_(A) with 1<c_(A)<q. Then CA computes γ_(A)=α^(c) ^(A)         mod p. (γ_(A) is A's public key reconstruction public data.         (I_(A), γ_(A)) serves as A's implicit certificate)     -   2. CA computes ƒ=F(γ_(A), I_(A), OP) and solves the following         equation for a (if a=0, 1, c, c_(A) ⁻¹c , then chooses another         c_(A) and re-solve the equation).         1=cƒ+c _(A)a (mod q).     -   3. CA securely sends the triple (γ_(A), a, I_(A)) to A, which is         CA's signature on I_(A). Then a is A's private key, γ_(A) is A's         generator and γ_(A) ^(a) (=α^(c) ^(A) ^(a)) is A's public key. A         publishes (α, I_(A), β, γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly verified public         key from the public domain by computing         γ_(A) ^(a)=αβ^(−ƒ) (mod p)

Note:

-   -   1. In step 1, The identity I_(A) may be chosen by entity A.     -   2. In step 2, we exclude a=0, 1, since in this case any one can         easily knowing A's private key. Especially when a=0, c_(A) ⁻¹c,         any one can compute CA's private key c from 1=cf (mod q).     -   3. For this scheme, each user has different system generator         γ_(A).         Scheme 1.b:     -   1. For each entity A, CA chooses a unique distinguished name or         identity I_(A) (e.g., name, address, phone number), and a random         integer c_(A) with 1<c_(A)<q . Then CA computes γ_(A)=α_(c) ^(A)         mod p. (γ_(A) is A's public key reconstruction public data.         (I_(A), γ_(A)) serves as A's implicit certificate)     -   2. CA computes ƒ=F(γ_(A), I_(A), OP) and solves the following         equation for a (if a=0, 1, c, then chooses another c_(A) and         re-solve the equation).         1≡ca+c _(A)ƒ (mod q).     -   3. CA securely sends the triple (γ_(A), α, I_(A)) to A, which is         CA's signature on I^(A). Then a is A's private key, β is A's         generator and β^(a) is A's public key. A publishes (α, I_(A), β,         γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly verified public         key from the public domain by computing         β^(a)=αγ_(A) ^(−ƒ) (mod p)

Note:

-   -   1. In step 1, the identity I_(A) may be chosen by entity A.     -   2. In step 2, we exclude a=0, 1, since in this case any one can         easily knowing A's private key, when a=0, the certificate does         not involve to CA.     -   3. For this scheme, each user has same system generator β.         Scheme 1.c:     -   1. For each entity A, CA chooses a unique distinguished name or         identity I_(A) (e.g., name, address, phone number), and a random         integer c_(A) with 1<c_(A)<q. Then CA computes γ_(A)=α^(c) ^(A)         mod p. (γ_(A) is A's public key reconstruction public data.         (I_(A), γ_(A)) serves as A's implicit certificate)     -   2. CA computes ƒ=F(γ_(A), I_(A), OP) and solves the following         equation for a (if a=0, 1, c, then chooses another c_(A) and         re-solve the equation).         a≡cf+C _(A) (mod q).     -   3. CA securely sends the triple (γ_(A), a, I_(A)) to A, which is         CA's signature on I_(A). Then a is A's private key, α is A's         generator and α^(a) is A's public key. A publishes (α, I_(A), β,         γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly verified public         key from the public domain by computing         α^(a)=β^(ƒγ) _(A) (mod p)

Note:

-   -   1. In step 1, The identity I_(A) may be chosen by entity A.     -   2. In step 2, we exclude a=0, 1, since in this case any one can         easily knowing A's private key.     -   3. For this scheme, each user has same system generator α.         Scheme 1.d:     -   1. For each entity A, CA chooses a unique distinguished name or         identity I_(A) (e.g., name, address, phone number), and a random         integer c_(A) with 1<c_(A)<q. Then CA computes γ_(A)=α^(c) ^(A)         mod p. (γ_(A) is A's public key reconstruction public data.         (I_(A), γ_(A)) serves as A's implicit certificate)     -   2. CA computes ƒ=F(γ_(A), I_(A), OP) and solves the following         equation for a (if a=0, 1 or c, then chooses another c_(A) and         re-solve the equation).         a≡c _(A) ƒ+c (mod q).     -   3. CA securely sends the triple (γ_(A), a, I_(A)) to A, which is         CA's signature on I_(A). Then a is A's private key, α is A's         generator and α^(a) is A's public key. A publishes (α, I_(A), β,         γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly verified public         key from the public domain by computing         α^(a)=γ_(A) ^(ƒ)β (mod p)

Note:

-   -   1. In step 1, The identity I_(A) may be chosen by entity A.     -   2. In step 2, we exclude a=0, 1 since in this case any one can         easily knowing A's private key.     -   3. For this scheme, each user has same system generator α.

Although everyone can reconstruct user A's public key from public data, this does not mean that the reconstructed public key has been certified. To explicitly verify the certificate, we need to know the a. Once we know the a, the verification process become to verify CA's signature on I_(A). For example, In scheme 1.a, if verifier computes αβ^(−ƒ) and user A computes γ_(A) ^(a) using a, then they can verify the certificate together. But verifier must make sure that user A indeed knows a. So reconstructing public key serves as an implicit verification only if it combines with an application protocol that shows user A has a complete knowledge of the corresponding private key. In general, the implicit certificate scheme can be used with any public key scheme which needs to authenticate the subject entity and the public key.

Let's demonstrate it by using DSA signature scheme as implicit certified public key system and scheme 1.a as implicit certificate scheme.

Suppose Alice has private key a, generator γ_(A) and publishes (α, I_(A), γ_(A), p, q) in public domain. Now Alice wants to sign a message M using DSA.

Alice does following:

-   -   1. randomly chooses k, computes r=γ_(A) ^(a) (mod p).     -   2. computes e=sha-1(M).     -   3. compute s=x⁻¹(e+ar) (mod q)     -   4. The signature on M is (r, s).

Verifier does following:

-   -   1. gets Alice's public data (α, I_(A), β, p, q) and computes ƒ         and reconstructs the public key         β_(A)=γ_(A) ^(a)=αβ^(−ƒ) (mod p)     -   2. computes e=sha-1(M).     -   3. computes u₁=es⁻¹ (mod q) and u₂=rs⁻¹ (mod q)     -   4. computes r′=γ_(A) ^(u) ¹ δ_(A) ^(u) ¹ (mod p)     -   5. if r=r′, the signature is verified. At same time Alice's         (ID-bases) public key is implicitly verified.

The pair (I_(A), γ_(A)) serves as certificate of Alice. For DSA, we know that it is very hard to forge Alice's signature without knowing a. Then reconstructing the public key serves as implicitly verification when the application protocol ends up with valid. Recall that obtaining the public key needs only one exponentiation operation. For this reason, we say that verifying the implicit certificate needs one exponentiation operation.

The following implicit certificate schemes may be derived by modifying the schemes above such that CA and entity both control the entity's private key but only the subject entity knows his/her private key.

In this section we need another system parameter H(*), where H(*) is an cryptographic function which may be a secure hash function or one way function or identity map.

Scheme 2.a:

-   -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes γ_(A)=α^(kc)         ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP), solves the signing         equation for k_(A) (if k_(A)=0 or c, then chooses another c_(A))         1=cƒ+c _(A) k _(A) (mod q).

Then CA computers γ_(A) ¹=α^(r) ^(A) (mod p) and sends the triple (γ_(A) ¹, k_(A), I_(A)) to A.

-   -   3. A computes a=k_(A)k⁻¹ (mod q). (if a=1, then goes back to         step 1.) and computes γ_(A)=(γ_(A) ¹)^(k) (mod p). Then checks         if γ_(A) ^(a)=αβ^(−ƒ). Now a is A's private key, γ_(A) is A's         generator and γ_(A) ^(a) is A's public key. A publishes (α,         I_(A), β, γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         γ_(A) ^(a)=αβ^(−ƒ) (mod p)         Scheme 2.b:     -   5. A randomly chooses an integer k and computes β^(k), then         sends it to CA.     -   6. CA randomly chooses an integer c_(A), computes         γ_(A)=β^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP), solves         the signing equation for k_(A) (if k_(A)=0, c, then chooses         another c_(A))         1=ck _(A) +c _(A)ƒ (mod q).

Then CA computers γ_(A) ¹=(β¹)^(c) ^(A) ^(c) ⁻¹ (mod p) and sends the triple (γ_(A) ¹, k_(A), I_(A)) to A.

-   -   7. A computes γ_(A)=(γ_(A) ¹)^(k) ⁻¹ β^(k) (mod p), ƒ=F(γ_(A),         I_(A), OP), and a=k_(A)−kƒ (mod q). (if a=0, 1, then goes back         to step 1.). Then checks if β^(a)=αγ_(A) ^(−ƒ). Now a is A's         private key, β is A's generator and β^(a) is A's public key. A         publishes (α, I_(A), β, γ_(A), p, q) in public domain.     -   8. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         β^(a)=αγ_(A) ^(−ƒ) (mod p)         Scheme 2.c:     -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP),         computes k_(A) (if k_(A)=c, then chooses another c_(A))         k _(A) ≡cƒ+c _(A) (mod q).

Then CA computes γ_(A) ¹=(α^(k))^(c) ^(A) (mod p) and sends the triple (γ_(A) ¹, k_(A), I_(A)) to A.

-   -   3. A computes a=k_(A)+k (mod q). (if a=0, 1, then goes back to         step 1.) and computes γ_(A)=(γ_(A) ¹)^(k) ⁻¹ α^(k) (mod p). Then         checks if an α^(a)=β^(ƒ)γ_(A). Now a is A's private key, α is         A's generator and α^(a) is A's public key. A publishes (α,         I_(A), β, γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=β^(ƒ)γ_(A) (mod p)         Scheme 2.d:     -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP),         computes k_(A) (if k_(A)=c_(A), then chooses another c_(A))         k _(A) ≡c _(A) ƒ+c (mod q).

Then CA computers γ_(A) ^(k)=(α^(k))^(c) ^(A) (mod p) and sends the triple (γ_(A) ¹, k_(A), I_(A)) to A.

-   -   3. A computes γ_(A)=(γ_(A) ¹)^(k) ⁻¹ α^(k) (mod p), ƒ=F(γ_(A),         I_(A), OP), and a=k_(A)+kƒ (mod q). (if a=0, 1, then goes back         to step 1.). Then checks if α^(a)=γ_(A) ^(ƒ)β. Now a is A's         private key, α is A's generator and α^(a) is A's public key. A         publishes (α, I_(A), β, γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=γ_(A) ^(ƒ)β (mod p)

Notes: (for scheme 2.a, 2.b, 2.c, 2.d)

-   -   1. The identity I_(A) may be chosen either by CA or by entity A     -   2. CA should authenticate the entity A. It can be done either by         presence in front of CA or by secure channel or by voice (for         example, on the phone) or by following method: In step 2,         instead of sending the triple (γ_(A) ¹, k_(A), I_(A)) to A, CA         first sends γ_(A) ¹ to A. A computes γ_(A), set K=H(γ_(A)),         encrypts the authentication information A_(A1) of A (such as         VISA information) by DES (or other symmetric key system) and         sends DES_(K)(A_(A1)) to CA. CA decrypts the DES_(K)(A_(A1)) to         get A_(A1). After checks the validity of A_(A1), CA then sends         (k_(A), I_(A)) to A.     -   3. (γ_(A) ¹, k_(A), I_(A)) can be sent by public channel.

In above scheme 2.a-2.d, The implicit certificate schemes are finished by the subject entity and the CA. Each scheme is essentially divided into two part: key-exchange part and signature part. One function of the key exchange part is to transmit implicit certificate information from CA to A by public channel (more discuss will be given in section 6). To speed up computation of above schemes, we can modify the key exchange part. Following scheme 3.a-3.d by modifying scheme 2.a-2.d. The advantages in scheme 3.a-3.d is that user A can compute K before he get respond from the CA since β is fixed. This property is good especially for the online case.

Scheme 3.a:

-   -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes γ_(A)=α^(kc)         ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP), solves the signing         equation for k_(A) (if k_(A)=0, then choose another c_(A))         1=cƒ+c _(A) k _(A) (mod q).

Next CA computers K=H((α^(k))^(c)) and k _(A)=DES_(K)(k_(A)), then sends the triple (γ_(A), k _(A), I_(A)) to A.

-   -   3. A computes K=H(β^(k)), k_(A)=DES_(K)( k _(A)), and a=k_(A)k⁻¹         (mod q). (if a=1, (then goes back to step 1.). Then checks if         γ_(A) ^(a)=αβ^(−ƒ). Now a is A's private key, γ_(A) is A's         generator and γ_(A) ^(a) is A's public key. A publishes (α,         I_(A), β, γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         γ_(A) ^(a)=αβ^(−ƒ) (mod p)         Scheme 3.b:     -   1. A randomly chooses an integer k and computes β^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes         γ_(A)=β^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP), solves         the signing equation for k_(A) (if k_(A)=0, then choose another         c_(A))         1=ck _(A) +c _(A)ƒ (mod q).

Next CA computers K=H((β^(k))^(c) ^(A) ^(c) ⁻¹ )=H(α^(kc) ^(A) ) and k _(A)=DES_(K)(k_(A)), then sends the triple (γ_(A), k _(A), I_(A)) to A.

-   -   3. A computes K=H((γ_(A)/β^(k))^(k))=H(α^(kc) ^(A) ),         k_(A)=DES_(K)( k _(A)), ƒ=F(γ_(A), I_(A), OP) and computes         a=k_(A)−kƒ (mod q). (if a=0, 1, then goes back to step 1). Then         checks if β^(a)=αγ_(A) ^(−ƒ). Now a is A's private key, β is A's         generator and β^(a) is A's public key. A publishes (α, I_(A), β,         γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         β^(a)=αγ_(A) ^(−ƒ) (mod p)

Note: (for scheme 3.b))

-   -   1. The identity I_(A) may be chosen either by CA or by entity A     -   2. CA should authenticate the entity A. It can be done either by         presence in front of CA or by secure channel or by voice (for         example, on the phone) or by following method: In step 2,         instead of sending the triple (γ_(A), k _(A), I_(A)) to A, CA         first sends γ_(A) to A. A computes         K=H((γ_(A)/β^(k))^(k))=H(α^(kc) ^(A) ), encrypts the         authentication information A_(A1) of A (such as VISA         information) by DES (or other symmetric key system) and sends         DES_(K)(A_(A1)) to CA. CA decrypts the DES_(K)(A_(A1)) to get         A_(A1). After checks the validity of A_(A1), CA then sends ( k         _(A), I_(A)) to A.     -   3. (γ_(A), k _(A), I_(A)) can be sent by public channel.         Scheme 3.c:     -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP),         computes k_(A) (if k_(A)=0, then choose another c_(A))         k _(A) ≡cƒ+c _(A) (mod q).

Next CA computers K=H((α^(k))^(c)) and k _(A)=DES_(K)(k_(A)), then sends the triple (γ_(A), k _(A), I_(A)) to A.

-   -   3. A computes K=H(β^(k)), k_(A)=DES_(K)( k _(A)), and a=k_(A)+k         (mod q) (if a=0, 1, then goes back to step 1.) Then checks if         α^(a)=β^(ƒ)γ_(A). Now a is A's private key, α is A's generator         and α^(a) is A's public key. A publishes (α, I_(A), β, β_(A),         p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=β^(ƒ)γ_(A) (mod p)         Scheme 3.d:     -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP),         computes k_(A) (if k_(A)=0, then choose another c_(A))         k _(A) ≡c _(A) ƒ+c (mod q).

Next CA computers K=H((α^(k))^(c)) and k _(A)=DES_(K)(k_(A)), then sends the triple (γ_(A), k _(A), I_(A)) to A.

-   -   3. A computes K=H(β^(k)), k_(A)=DES_(K)( k _(A)), ƒ=F(γ_(A),         I_(A), OP), and a=k_(A)+kƒ (mod q). (if a=0, 1, then goes back         to step 1.). Then checks if α^(a)=γ_(A) ^(ƒ)β. Now a is A's         private key, α is A's generator and α^(a) is A's public key. A         publishes (α, I_(A), β, γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=γ_(A) ^(ƒ)β (mod p)

Notes: (for scheme 3.a, 3.c, 2.d)

-   -   1. The identity I_(A) may be chosen either by CA or by entity A     -   2. CA should authenticate the entity A. It can be done either by         presence in front of CA or by secure channel or by voice (for         example, on the phone) or by following method: In step 1, A         compute α^(k) and K=H(β^(k)), then sends α^(k) and         DES_(K)(A_(A1)) to CA. CA computes K=H((α^(k))^(c)) and decrypts         the DES_(K)(A_(A1)) to get A_(A1). After check the validity of         A_(A1), CA continues step 2.     -   3. (γ_(A), k_(A), I_(A)) can be sent by public channel.

The advantages for scheme 3.a, 3.c and 3.d are that user A can compute K easily since β is fixed and that k_(A) is encrypted such that no other people can know it. In fact the publicity of k_(A) does not decrease the security of the certificate scheme. The purpose of encrypting k_(A) is to make sure that the entity knows k. So for scheme 3.a-3.d, the DES encryption part can be removed and k _(A) can be replaced by k_(A) provided the certificate scheme uses the method described in Note 2.

To save transmission bandwidth in above schemes, we can modify above schemes by sending ƒ=F(γ_(A), I_(A), OP) instead of γ_(A) (Note that in general, the size of γ_(A) is large than 160 bits and ƒ is just 160 bits.) Following scheme 4.c is a modification of scheme 3.c.

Scheme 4.c:

-   -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A), OP),         computes k_(A) (if k_(A)=0, then choose another c_(A))         k _(A) ≡cƒ+c _(A) (mod q).

Next CA computers K=H((α^(k))^(c)) and k _(A)=DES_(K)(k_(A)), then sends the triple (ƒ, k _(A), I_(A)) to A.

-   -   3. A computes K=H(β^(k)), k_(A)=DES_(K)( k _(A)), and a=k_(A)+k         (mod q) (if a=0, 1, then goes back to step 1.) Then computes         γ_(A)=α^(a) β^(−ƒ) (mod p) and checks if ƒ=F(γ_(A), I_(A), OP).         Now a is A's private key, α is A's generator and α^(a) is A's         public key. A publishes (α, I_(A), β, γ_(A), p, q) in public         domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=β^(ƒ)γ_(A) (mod p)

Furthermore we can reduce the bandwidth by following scheme 5.c.

Scheme 5.c:

-   -   1. A randomly chooses an integer k and computes α^(k), then         sends it to CA.     -   2. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and set {circumflex over (γ)}_(A)         as the first 80 least significant bits of γ_(A). Then computes         ƒ=F({circumflex over (γ)}_(A), I_(A), OP) and k_(A) (if k_(A)=0,         then choose another c_(A))         k _(A) ≡cƒ+c _(A) (mod q).

Next CA computers K=(α^(k))^(c) (mod p) and k _(A)=DES_(K)(k_(A)), then sends the triple ({circumflex over (γ)}_(A), {circumflex over (k)}_(A), I_(A)) to A.

Note: ({circumflex over (γ)}_(A), k _(A), I_(A)) can be sent by public channel.

-   -   3. A computes K=β^(k) (mod p), k_(A)=DES_(K)( k _(A)), and         a=k_(A)+k (mod q) (if a=0, 1, then goes back to step 1.) Then         computes ƒ=({circumflex over (γ)}_(A), β, I_(A)), γ_(A)=α^(a)         β^(−ƒ) (mod p) and checks if the first 80 least significant bits         of γ_(A) is {circumflex over (γ)}_(A). Now a is A's private key,         α is A's generator and α^(a) is A's public key. A publishes (α,         I_(A), β, γ_(A), p, q) in public domain.     -   4. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=β^(ƒ)γ_(A) (mod p)

The security level of scheme 5.c, is not as other schemes we discuss before. Scheme 5.c only has 80 bit security. But it is OK for practical application Now. We can extend the first 80 least significant bits to the half least significant bits of γ_(A).

The implicit certificate can be used to certify some other useful information by including the information in the option parameter OP. For example OP=α^(a) ^(E) ∥OP₂, where a_(E) is user A's another private key and α^(a) ^(E) is the corresponding public key. Following scheme 6.c is a modification of scheme 2.c. Other schemes can be modified in the same way.

Scheme 6.c:

-   -   1. A randomly chooses an integer a_(E) and computes α^(a) ^(E) .     -   2. A randomly chooses an integer k and computes α^(k), then         sends α^(k) and α^(a) ^(E) to CA.     -   3. CA randomly chooses an integer c_(A), computes         γ_(A)=α^(k)α^(c) ^(A) (mod p) and ƒ=F(γ_(A), I_(A), α^(a) ^(E) ,         OP₂) (for example, F(γ_(A), I_(A), α^(a) ^(E) ,         OP₂)=h(γ_(A)∥I_(A)∥α^(a) ^(E) )), computes k_(A) (if k_(A)=0,         then choose another c_(A))         k _(A) ≡cƒ+c _(A) (mod q).

Then CA computers γ_(A) ¹=(α^(k))^(c) ^(A) (mod p) and sends the triple (γ_(A) ¹, k_(A), I_(A)) to A.

-   -   4 A computes a=k_(A)+k (mod q). (if a=0, 1, then goes back to         step 1.) and computes γ_(A)=(γ_(A) ¹)^(k) ⁻¹ α^(k) (mod p). Then         checks if α^(a)=β^(ƒ)γ_(A). Now a is A's private signing key, α         is A's generator and α^(a) is A's public signing key. a_(E) is         A's private encryption key and α^(a) ^(E) is A's public         encryption key. A publishes (α, α^(a) ^(E) , I_(A), β, γ_(A),         p, q) in public domain.     -   5. Anyone can obtain A's (ID-based) implicitly certified public         key from the public domain by computing         α^(a)=β^(ƒ)γ_(A) (mod p).

Notes: (for scheme 4.c, 5.c, 6.c)

-   -   1. The identity I_(A) may be chosen either by CA or by entity A     -   2. CA should authenticate the entity A. It can be done by the         method described in the note 2 of scheme 3.c. (ƒ, k _(A), I_(A))         or ({circumflex over (γ)}_(A), k _(A), I_(A)) or (γ_(A) ¹,         k_(A), I_(A)) can be sent by public channel.         CA Chaining Scheme         In order to implement a CA chaining structure. That is CA1         authenticates CA2, CA2 authenticates CA3 and CA3 authenticates         user A. In this section, we are going to present the example         with 3 CA's in the CA chain. We use basic scheme 3′ to         demonstrate this example.

System Setup:

The highest trusted party CA1 selects an appropriate prime p with p=tq+1 where q is a large prime and a generator α of order q. CA1 selects a random integer c₁, with 1≦c₁≦q−1 as its private key, then computes the public key β₁α^(c) ¹ mod p and publishes (β₁, α, p, q).

Phase 1. CA2 applies for Implicit Certified Public Key from CA1.

-   -   1. CA2 randomly chooses an integer k₂ and computes α^(k) ¹ ,         then sends it to CA1.     -   2. CA1 choose a unique distinguished name or identity I_(CA2)         and a random integer c_(CA2) with 1≦c_(CA2)≦q−1. Then CA1         computes γ_(CA2)=α^(k) ² α^(r) ^(CA1) (mod p). (β_(CA2) is CA2's         public key reconstruction public data.)     -   3. CA1 chooses a function ƒ₁=F(γ_(CA2), I_(CA2)) and computes         k_(CA2) (if k_(CA2)=0, then chooses another c_(CA2) in step 2         and re-computes for k_(CA2)).         k _(CA2) ≡c ₁ƒ₁ +c _(CA2) (mod q)     -   4. CA1 computes γ_(CA2) ¹=(α^(k) ² )^(c) ^(CA2) (mod p) and         sends the triple (γ_(CA2) ¹, k_(CA2), I_(CA2)) to CA2.     -   5. CA2 computes γ_(CA2)=(γ_(CA2) ¹)^(k) ² ⁻¹ α^(k) ² (mod p).         Then c₂=k_(CA2)k₂ (mod q) is CA2's private key, α is CA2's         generator and β₂=α^(r) ¹ is CA2's public key. CA2 publishes (α,         I_(CA2), β₁, β₂, γ_(CA2), p, q) in public domain.

Note: when a user trusts CA2, be/she can use β₂ directly.

-   -   6. Anyone can obtain CA2's (ID-based) implicitly verified public         key from the public domain by computing         β₂=α^(r) ¹ =β₁ ^(ƒ) ² γ_(CA2) (mod p)         Phase 2. CA3 Applies for Implicit Certified Public Key from CA2.     -   1. CA3 randomly choose an integer k₃ and computes α^(k) ³ , then         sends it to CA2.     -   2. CA2 choose a unique distinguished name or identity I_(CA3)         and a random integer c_(CA3) with 1≦c_(CA3)≦q−1. Then CA2         computes γ_(CA3)=α^(k) ³ α^(c) ^(CA3) (mod p). (γ_(CA3) is CA3's         public key reconstruction public data.)     -   3. CA2 chooses a function ƒ₂=F(γ_(CA3), I_(CA3)) and computes         k_(CA3) (if k_(CA3)=0, then chooses another c_(CA3) in step 2         and re-computes for k_(CA3)).         k _(CA3) ≡c ₃ƒ₂ +c _(CA3) (mod q)     -   4. CA2 computes γ_(CA3) ¹=(α^(k) ³ )^(c) ^(CA3) (mod p) and         sends the triple (γ_(CA3) ¹, k_(CA3), I_(CA3)) to CA3.     -   5. CA3 computes γ_(CA3)=(γ_(CA3) ¹)^(k) ³ ⁻¹ α^(k) ³ (mod p).         Then c₃=k_(CA3)+k₃ (mod q) is CA3's private key, α is CA3's         generator and β₃=α^(c) ³ is CA3's public key. CA3 publishes (α,         I_(CA3), β₂, β₃γ_(CA3), p, q) in public domain.

Note: when an entity trusts CA3, it can use β₃ directly.

-   -   6. Anyone can obtain CA3's (ID-based) implicitly verified public         key from the public domain by computing         β₃=α^(c) ³ =β₂ ^(ƒ) ³ γ_(CA3) (mod p)         Phase 3. User A Applies for Implicit Certified Public Key From         CA3.     -   1. A randomly choose an integer k and computes α^(k), then sends         it to CA3.     -   2. CA3 choose a unique distinguished name or identity I_(A) and         a random integer c_(A) with 1≦c_(A)≦q−1. Then CA3 computes         γ_(A)=α^(k)α^(c) ^(A) (mod p). (γ_(A) is A's public key         reconstruction public data.)     -   3. CA3 choose a careful chosen function ƒ₃=F(γ_(A), I_(A)) and         computes k_(A) (if k_(A)=0, then choose another c_(A) in step 2         and re-computes for k_(A)).         k _(A) ≡c ₃ƒ₃ +c _(A) (mod q)     -   4. CA3 computes γ_(A) ¹=(α^(k))^(c) ^(A) (mod p) and sends the         triple (γ_(A) ¹, k_(A), I_(A)) to A.     -   5. A computes γ_(A)=(γ_(A) ¹)^(k) ⁻¹ α^(k) (mod p). Then         a=k_(A)+k (mod q) is A's private key, α is A's generator and         β_(A)=α^(a) is A's public key. A publishes (α, I_(A), β₃, β_(A),         γ_(A), p, q) in public domain.

Note: when a user trusts A, he/she can use β_(A) directly.

-   -   6. Anyone can obtain A's (ID-based) implicitly verified public         key from the public domain by computing         β_(A)=α^(a)=β₃ ^(ƒ) ³ γ_(A) (mod p)         Phase 4. User A's Signature and Verification.

To sign a message M, user A does following:

-   -   1. randomly choose x, computes r=α^(x) (mod p).     -   2. computes e=ƒ_(A)=F(r, M), where F is some fixed function.     -   3. computes s=ae+x (mod q)     -   4. The signature on M is (r, s).

Verifier does following:

-   -   1. gets CA1, CA2, CA3 and User A's public data         (α, I_(CA2), I_(CA3) I_(A), β₁, β₂, β₃, β_(A), γ_(CA2), γ_(CA3),         γ_(A), p, q)     -   2. reconstructs user A's public key         β_(A)=β^(ƒ1ƒ2ƒ3)γ_(CA) ^(ƒ2ƒ3) _(CA)/_(CA) ^(ƒ3)γ_(A) (mod p)     -   3. computes e=ƒ_(A)=F(r, M).     -   4. computes r′=α¹β_(A) ^(−e) (mod p)     -   5. if r=r′, the signature is verified. At same time CA2, CA3 and         user A's (ID-bases) public key are implicitly verified.

Reconstructing user A's public key needs only 3 known basis exponentiation operations and 3 multiplication operations. When the signature is valid, CA2, CA3 and user A's (ID-bases) public key are implicitly verified.

Notes:

-   -   1. If verifier trusts A, Then A's public key is β_(A).     -   2. If verifier trusts CA3, Then A's reconstruction public key is         β_(A)=β₃ ^(ƒ) ³ γ_(A) (mod p)     -   3. If verifier trusts CA2, Then A's reconstruction public key is         β_(A)=β₂ ^(ƒ) ^(1ƒ) ² γ_(CA3) ^(ƒ) ³ γ_(A) (mod p)         Co-signing Scheme.

The following describes a scheme that allows multiple CA's to sign ONE implicit certificate. This is illustrated by the case where three CA's co-sign a certificate using the basic scheme 3′.

System Setup:

Let CA1, CA2 and CA3 have a common system parameters; (1) prime p with p=tq+1 where q is a large prime; (2) a generator α of order q; (3) a careful chosen function ƒ=F(γ,(I_(A1)+I_(A2)+I_(A3))). CA1 selects a random integer c₁, with 1≦c₁≦q−1 as its private key, then computes the public key β₁=α^(c) ¹ mod p and publishes (β₁, α, p, q). CA2 selects a random integer c₂, with 1≦c₂≦q−1 as its private key, then computes the public key β₂=α^(c) ¹ mod p and publishes (β₃, α, p, q). CA3 selects a random integer c₃, with 1≦c₃≦q−1 as its private key, then computes the public key β₃=α^(c) ³ mod p and publishes (β₃, α, p, q).

-   Step 1. A randomly chooses an integer k and computes α^(k), then     sends it to CA1, CA2 and CA3. -   Step 2. CA's exchange information and compute implicit certificates     Phase 1.     -   1. CA1 chooses a unique distinguished name or identity I_(A1)         and a random integer c_(A1) with 1≦c_(A1)≦q−1, computes α^(c)         ^(A1) and send (α^(c) ^(A1) , I_(A1)to CA2, and CA3.     -   2. CA2 choose a unique distinguished name or identity I_(A2) and         a random integer c_(A2) with 1≦c_(A2)≦q−1, computes (α^(c) ^(A2)         , I_(A2)) and send a^(c) ^(A2) to CA1 and CA3.     -   3. CA3 choose a unique distinguished name or identity I_(A3) and         a random integer c_(A3) with 1≦c_(A3)≦q−1, computes (α^(c) ^(A3)         , I_(A3)) and send α^(c) ^(A3) to CA1 and CA2.         Phase 2.     -   1. CA1 computes γ=α^(k)α^(c) ^(A1) α^(c) ^(A2) α^(c) ^(A3) (mod         p). (γis A's public key reconstruction public data.), computes         ƒ=F(γ,(I_(A1)+I_(A2)+I_(A3))) and computes k_(A1) (if k_(A1)=0,         then goes back to phase 1.)         k _(A1) ≡c ₁ ƒ+c _(A1) (mod q)

CA1 computes γ_(A1) ¹=(α^(k))^(c) ^(A1) (mod p) and sends the triple (γ_(A1) ¹, k_(A1), I_(A1)) to A.

-   -   2. CA2 computes γ=α^(k)α^(c) ^(A1) α^(c) ^(A2) α^(c) ^(CA3) (mod         p). (γ is A's public key reconstruction public data.), computes         ƒ=F(γ,(I_(A1)+I_(A2)+I_(A3))) and computes k_(A2) (if k_(A2)=0,         then goes back to phase 1.)         k _(A2) ≡c ₂ ƒ+c _(A2) (mod q)

CA2 computes γ_(A2) ¹=(α^(k))^(c) ^(A2) (mod p) and sends the triple (γ_(A2) ¹, k_(A2), I_(A2)) to A.

-   -   3. CA3 computes γ=α^(k)α^(c) ^(A1) α^(c) ^(A2) α^(c) ^(CA3) (mod         p). (γ is A's public key reconstruction public data.), computes         ƒ=F(γ,(I_(A1)+I_(A2)+I_(A3))) and computes k_(A3) (if k_(A3)=0,         then goes back to phase 1.)         k _(A3) ≡c ₁ ƒ+c _(A3) (mod q)

CA3 computes γ_(A3) ¹=(α^(k))^(c) ^(A3) (mod p) and sends the triple (γ_(A3) ¹, k_(A3), I_(A3)) to A.

-   Step 3. A computes implicitly co-certified private keys and public     key reconstruction information.     -   1. A computes a=k_(A1)+k_(A2)+k_(A3)+k (mod q). (If a is 0 or 1,         then goes back to step 1.)     -   2. A computes γ=(γ_(A1) ¹γ_(A2) ¹γ_(A3) ¹)^(k) ⁻¹ α^(k) (mod p),         ƒ=F(γ,(I_(A1)+I_(A2)+I_(A3))). Then verifies if         α^(a)=(β₁β₂β₃)^(ƒ)γ (mod p).     -   3. Then a is A's implicitly co-certified private key, α is A's         generator, I_(A)=I_(A1)+I_(A2)+I_(A3) is A's common ID and         (β₁β₂β₃)^(ƒ)γ is A's implicitly co-certified public key.     -   4. A publishes (α, I_(A1), I_(A2), I_(A3), β₁, β₂, β₃, γ, p, q)         in public domain.     -   5. Anyone can obtain A's (ID-based) implicitly co-certified         public key from the public domain by computing (β₁β₂β₃)^(ƒ) γ         (mod p)         Applications

The following examples are illustrated with respect to scheme 3 (or Scheme 7′) as CA's signing equation since everyone shares the same generator in this scheme. Each user can have a different CA as long as the CAs use the system parameters (p, q, d) and each user has the same generation.

Setup:

-   CA1: system parameters (α, β₁, p, q, d)

Alice has a private key a, generator α and publishes (α, I_(A), β, γ_(A), p, q) in the public domain.

-   CA2: system parameters (α, β₂, p, q)

Bob has a private key b, a generator α and publishes (α, I_(A), β, γ_(A), p, q) in the public domain.

We use the MTI/C0 key agreement protocol to demonstrate how to use our new scheme. Assume Alice and Bob want to perform a key exchange.

The MTI/C0 Protocol

-   -   1. Alice reconstructs Bob's public key α^(b)=β^(F(γ) ^(A) ^(, I)         ^(A) ⁾γ_(B), and randomly chooses an integer x and computes         (α^(b))^(x), then sends it to Bob.     -   2. Bob reconstructs Alice's public key α^(a)=β^(F(γ) ^(A) ^(, I)         ^(A) ⁾γ_(A), and randomly chooses an integer y and computes         (α^(a))^(y), then sends it to Alice.     -   3. Alice computes the shared key K_(A)=(α^(ay))^(xa) ⁻¹ =α^(xy)     -   4. Bob computes the shared key K_(B)=(α^(bx))^(yb) ⁻¹ =α^(xy)

This is a two-pass protocol. With the implicit certificate scheme of the present invention, each party only does three exponentiation operations to get the shared key while at the same time performing an authentication key agreement and implicit public key verification.

The following are examples of signcryption schemes. We use scheme 3 (or scheme 7) as CA's signing equation since everyone shares the same generator in this scheme. For the scheme thereafter, we use scheme 13 as CA's signing equation. For all schemes in this section, each user can have a different CA as long as the CA's use the same system parameters (p, q, α) and each user has the same generator.

Setup:

-   CA1: system parameters (α, β₁, p, q)

Alice: private key a, generator α and (α, I_(A), β₁, γ_(A), p, q) in public domain.

-   CA2: system parameters (α, β₂, p, q)

Bob: private key b, generator α and (α, I_(B), β₂, γ_(B), p, q) in public domain.

Bob wants to send a signed and encrypted message M to Alice:

Signcryption Protocol 1:

Assume Bob wants to send a signed and encrypted message M to Alice:

Bob does following:

-   -   1. reconstructs Alice's public key α^(a)=β^(F(γ) ^(A) ^(, I)         ^(A) ⁾γ_(A) mod p     -   2. randomly chooses an integer x and computes a key         r=(α^(a))^(x) (mod p)     -   3. computes C=DES_(r)(M)     -   4. computes e=hash(C I_(A))     -   5. computes s=be+x (mod q)     -   6. sends the pair (C, s) to Alice, thus C is the encrypted         message and s is the signature.

To recover the message Alice does following:

-   -   1. computes e=hash(C I_(A))     -   2. reconstructs Bob's public key α^(b)=β^(F(γ) ^(B) ^(, I) ^(B)         ⁾γ_(B) mod p     -   3. computes α^(as)(α^(b))^(−ac) (mod p) which is r     -   4. decrypts the message M=DES_(r)(C)     -   5. check for redundancy

Thus, Bob only does two exponentiation operations and Alice does three exponentiation operations. But Alice and Bob are both confident of each others authentication. Note that for this scheme, the message M must have some redundancy or pattern.

Signcryption Protocol 2 (General Case):

Setup:

-   CA1: system parameters (α, β₁, p, q)

Alice: private key a, generator α and (α, I_(A), β₁, γ_(A), p, q) in public domain.

-   CA2: system parameters (α, β₂, p, q)

Bob: private key b, generator α and (α, I_(B), β₂, γ_(B), p, q) in public domain

Note: this set up is for implicit certificate. For usual certificate scheme systems, we only required that Alice and Bob has same generator.

To signcrypt a message to Alice, Bob does following:

-   -   1. gets Alice's public key α^(a) (in the case of implicit         certificate scheme, reconstructs Alice's public key α^(a)=β₁         ^(F(γ) ^(A) ^(, β) ² ^(, I) ^(A) ⁾γ_(A) (mod p))     -   2. random choose an integer x and computes r=(α^(a))^(x) (mod p)     -   3. computes C=DES_(r)(M)     -   4. computes e=hash(C∥α^(a))     -   5. computes s=be+x (mod q)     -   6. sends (C, s) to Alice. C is the encrypted message and s is         the signature.

To recover the message Alice does following:

-   -   1. computes e=hash(C∥α^(a))     -   2. gets Bob's public key α^(b) (in the case of implicit         certificate scheme, reconstructs Bob's public key α^(b)=β₂         ^(F(γ) ^(B) ^(, β) ^(B) ^(, I) ^(B) ⁾γ_(B) (mod p))     -   3. computes α^(a1)(α^(b))^(−ae) (mod p) which is r     -   4. decrypts the message AM=DES_(r)(C)

Note:

-   -   1. If the certificate scheme is not the implicit certificate as         described herein, Alice and Bob's public key should be verified.     -   2. The message M must have some redundancy or pattern.     -   3. Anyone who knows one value r can decrypt any messages from         Bob to Alice since the value α^(ab) will be exposed.     -   4. In general, we should include an option parameter to the hash         e, i.e. e=hash(C∥α^(a)∥OP). For example, OP=α^(b) or         OP=α^(b)∥β₁∥β₂.

The signcryption schemes above have a drawback that if the signer lost his/her private signing key, then all message the signer signcrypted will be exposed to public. To protect post encryption we propose a new signcryption scheme. In new scheme, each user has two pairs of key, one pair is for signature key, another pair is encryption key. The new scheme can be used with any certificate scheme. But if it is used with our implicit certificate scheme, it is more efficient.

Signcryption Protocol 3 (General Case):

Setup:

Alice: private signing key a and private encryption key a_(E), generator α and (α, α^(a) ^(E) , I_(A), β₁, γ_(A), p, q) in public domain.

-   CA2: system parameters (α, β₂, p, q)

Bob: private signing key b and private encryption key b_(E), generator α and (α, α^(b) ^(E) , I_(B), β₂, γ_(B), p, q) in public domain

Note: this set up is for implicit certificate using scheme 6.c. For usual certificate scheme systems, we only required that Alice and Bob has same generator.

To signcrypt a message to Alice, Bob does following:

-   -   1. gets Alice's public signing key α^(a) and public encryption         key α^(a) ^(E) (in the case of implicit certificate scheme,         reconstructs Alice's public signing key         α^(a)=β₁ ^(F(γ) ^(A) ^(, β) ¹ ^(, I) ^(A) ^(, α) ^(a) ^(E)         ⁾γ_(A) (mod p))     -   2. random choose an integer x and computes r=(α^(a)α^(a) ^(E)         )^(x) (mode p)     -   3. computes C=DES_(r)(M)     -   4. computes e=hash(C∥α^(a)∥α^(a) ^(E) ∥α^(b)∥α^(b) ^(E) ∥OP.)     -   5. computes s=be+x+b_(E) (mod q)     -   6. sends (C, s) to Alice. C is the encrypted message and s is         the signature.

To recover the message Alice does following:

-   -   1. computes e=hash(C∥α^(a)∥α^(a) ^(E) ∥α^(b)∥α^(b) ^(E) ∥OP)     -   2. gets Bob's public signing key α^(b) and public encryption key         α^(b) ^(E) (in the case of implicit certificate scheme,         reconstructs Bob's public sign key α^(b)=β₂ ^(F(γ) ^(B) ^(, β) ²         ^(, I) ^(B) ^(, α) ^(a) ^(E) ⁾γ_(B) (mod p))     -   3. computes α^((a+a) ^(E) ^()s)(α^(b))^(−(a+a) ^(E) ^()c)         α^(−(a+a) ^(E) ^()b) ^(E) (mod p) which is r     -   4. decrypts the message M=DES_(r)(C)

Note:

-   -   1. we can think the receiver Alice's private key is a+a_(E),         This means the receiver only needs one private key instead of         two private keys. But the sender Bob needs two private keys. In         case of normal certificate, the receiver only need one private         key.     -   2. If the certificate scheme is not the implicit certificate         described in this application, Alice and Bob's public key should         be verified.     -   3. The message M must have some redundant or pattern.     -   4. The parameter OP inside hash e=hash(C∥α^(a)∥α^(a) ^(E)         ∥α^(b)∥α^(b) ^(E) ∥OP) may be empty or OP=β₁∥β₂.     -   5. Knowing one r value does not reveal any information of the         post messages.     -   6. With implicit certificate scheme, Bob only does 2         exponentiation operations and Alice does 4 exponentiation         operations. But Alice and Bob both are confidential that each         other is authentication part.     -   7. If anyone knows Alice's private key a+a_(E), or Bob lost both         private keys, the post encrypted message can not be protected.

For normal signatures, one problem is that the signer denies he/she signs the signature. This called repudiation. Protocol 1 and 2 above have a non-repudiation feature provided one trusts the judge. That is the signer can not deny that he/she signed the signcrypted message. Protocol 3 has a non-repudiation feature even when the judge is not trusted. Next protocol demonstrates how a judge decides a case where Bob wants to deny the signature.

Non-repudiation Protocol:

-   -   1. Alice sends (C, s) to Judge     -   2. Judge computes e=hash(C∥α^(a)∥α^(a) ^(E) ∥α^(b)∥α^(b) ^(E)         ∥OP) and α^(s)=α⁵(α^(b))^(−e) α^(−b) ^(E) (Note: Alice and Bob's         two pairs of public key should be verified. In the case of         implicit certificate scheme, the public keys should be computed         from the reconstruction public data.)     -   3. Judge randomly chooses two integer r₁ and r₂ and computes         L=(α^(A))^(r) ¹ α^(r) ² and sends L to Alice     -   4. Alice computes L^(a+a) ^(E) and sends it back to Judge     -   5. Judge computes r=(L^((a+a) ^(E) ⁾(α^(a)α^(αa) ^(E) )^(−r) ¹         )^(r) ¹ ⁻¹ and recover the message by M=DES_(r)(C)     -   6. If M has proper format, the (C, s) must be signcrypted by         Bob.     -   7. After the judge make decision, he sends the values (α^(x),         r₁, r₂, L, L^(a+a) ^(E) , r) to Alice and Bob to back tip his         decision.

For the other two signcryption protocols the non-repudiation protocols are similar provided one fully trust the judge.

In conclusion it may be seen that the present scheme, when combined with an application protocol for which the user's private key must be used directly in computation, provides an implicitly certified ID-based public key of the user. These schemes can also be used for a Key Authentication Center (KAC) to distribute implicitly certified public keys to users.

A further application of implicitly certified public keys is that the bit strength of the certifying authority is the same as the user or entity public keys being certified. By bit strength it is implied the relative key sizes and processing power of the entities concerned.

One approach to addressing this issue is to embed implicitly certified public keys into more traditional certificate structures such as specified in X.509 certificates, where the signature on the certificate is at a higher bit strength than the implicitly certified public key. Hence, the CA has certified the user public key at two different security levels. Any other entity retrieving a public key can decide on which security level they wish to accept. In some applications it may be that only the lower level provided by the implicit value is necessary to provide the performance required.

While the invention has been described in connection with specific embodiments thereof and in specific uses, various modifications thereof will occur to those skilled in the art without departing from the spirit of the invention as set forth in the appended claims. For example in the above description of preferred embodiments, use is made of multiplicative notation, however the method of the subject invention may be equally well described utilizing additive notation. It is well known for example that elliptic curve algorithm embodied in the ECDSA is equivalent of the DSA and that the elliptic curve analog of a discrete log logarithm algorithm that is usually described in a setting of, F^(•) _(p) the multiplicative group of the integers modulo a prime. There is a correspondence between the elements and operations of the group F^(•) _(p) and the elliptic curve group E(F_(q)). Furthermore, this signature technique is equally well applicable to functions performed in a field defined over F_(p) and F_(2•). It is also to be noted that the DSA signature scheme described above is a specific instance of the ElGamal generalized signature scheme which is known in the art and thus the present techniques are applicable thereto. 

1. A cryptographic unit of an entity in a secure communication system for enabling at least one subscriber A to generate a public key, said cryptographic unit being configured for: a) selecting a unique identity I_(A) distinguishing said subscriber A; b) generating public key reconstruction data γ_(A) of said subscriber A by mathematically combining public values obtained from respective private values of said entity and said subscriber A, to obtain a pair (I_(A), γ_(A)) serving as said subscriber A's implicit certificate; c) combining said implicit certificate (I_(A), γ_(A)) in accordance with a mathematical function F(I_(A), γ_(A)) to derive entity information f, and d) generating a value k_(A) by binding said entity information f with private values of said entity and transmitting said value k_(A) to said subscriber A to permit said subscriber A to generate a private key from said value k_(A), the private value of said subscriber A, and said implicit certificate; whereby said subscriber A's public key may be reconstructed from public information, said public key reconstruction data γ_(A), and said identity I_(A).
 2. A cryptographic unit according to claim 1, wherein said mathematical function is a secure hash function.
 3. A cryptographic unit according to claim 1, wherein said private value of said subscriber A is made available at said subscriber A and the corresponding public value obtained therefrom is made available at said entity.
 4. A cryptographic unit according to claim 1, wherein the mathematical combination of said public values obtained from respective private values of said entity and said subscriber A is a multiplication.
 5. A cryptographic unit according to claim 1, wherein private values of said entity include a private key and an integer.
 6. A cryptographic unit according to claim 5, wherein one of the public values obtained from respective private values of said entity and said subscriber A, corresponds to said private key of said entity.
 7. A cryptographic unit according to claim 5, wherein said value k_(A) is computed by multiplying said entity information f by said integer and adding said private key of said entity thereto.
 8. A cryptographic unit according to claim 6, wherein said value k_(A) is computed by multiplying said entity information f by said integer and adding said private key of said entity thereto. 